Sodium high-pressure discharge lamps or metal halide discharge lamps frequently use ceramic discharge vessels, which permits an increased operating temperature over discharge vessels using glasses. Ceramic discharge vessels and melt-through systems are well known from sodium high-pressure discharge vessels. Frequently, the through-leads, extending from the outside of the discharge vessel into the discharge vessel itself, are made of niobium or tantalum, since these metals have thermal coefficients of expansion roughly similar to those of ceramic plugs. The lead-through elements, which may be tubular or solid, or rods with a thin internal bore, are melt-sealed by a glass melt in the ceramic end plugs--see, for example, the referenced British Patent 1,465,212, Rigden, and U.S. Pat. No. 4,376,905, Kerekes.
U.S. Pat. No. 4,545,799, Driessen et al, describes a sodium high-pressure lamp having a niobium current through-lead which is sealed by passing the niobium lead through a plug of "green" aluminum oxide (Al.sub.2 O.sub.3), and sintering the lead into the plug without glass melt. This process is possible since the materials being sintered together have roughly the same thermal coefficient of expansion, 8.times.10.sup.-6.degree. /K. The simple sintering technology can be used, however, only with tubular lead-through arrangements, since the natural elasticity of a comparatively thin-walled tube is utilized. It does not work with solid pins or rods, or pins or rods having only a very thin bore, since the necessary elasticity is absent, leading rapidly to leaks in the seal.
The use of a glass melt for a seal has a serious problem:
It is readily possible to seal one end of a discharge vessel without difficulties by use of a sealing glass or glass melt, even if the through-lead is a rod or pin. Before the second end is closed off, however, it is necessary to first introduce the fill into the discharge vessel. Then the subassembly of the lead-through is applied to the second end of the discharge vessel. A ring of glass seal or glass melt material is applied to the plug, externally of the vessel. It is now necessary to heat this ring of melt glass in order to liquefy the sealing glass, so that it runs in gaps which occur between the plug and the lead-through. Heating the second end, however, has an effect on the fill. It leads to an undesired increase of the pressure of the fill within the discharge vessel. This pressure tends to press the now liquefied sealing glass, as well as the lead-through itself, out of the plug, that is, outwardly of the vessel. It is possible to counteract this increased pressure effect by increasing the pressure at the outside of the vessel, when the second end is being sealed.
The control of the external pressure, that is, the pressure outside of the sealing vessel, must mimick the increase of the pressure within the discharge vessel as the sealing glass melts. This pressure increase requires careful observation and control, and the quality of the resulting melt seal depends decidedly on correctly increased pressure. This increase in pressure, frequently, is manually controlled, while observing the melting process, and requires a good deal of intuition on the part of the operator. This melt-in process is so complex that it could not be automated heretofore; a high reject rate due to foreshortened lifetime had to be contended with.